1. Field of the Invention
The present invention relates to an optical recording medium capable of recording and reproducing information signals to both of grooves and lands therebetween and more in particular to an optical recording medium capable of increasing capacity by increased track density.
2. Description of the Prior Art
In recent years, various rewritable type optical recording media capable of rewriting information have been studied as the optical recording media for recording, for example, processing data of a computer and audio or video digital signals. Particularly, since the amount of data becomes enormous, for example, in digitalization of video signals, it has been demanded more for increasing the capacity of information recording in optical recording media or apparatus therefor.
In view of the above, for coping with increasing capacity of the image recording, various attempts have been made at present such as shortening of laser wavelength (Yamanaka, et. al; "Recording/Erasing Characteristic of GeSbTe System Phase Change Optical Disc by a Light at 488 nm", in '93 Phase Change Optical Disc Symposium, pp 75), "Super Resolution Optical Recording System" (Yamanaka, etc.; Jpn. J. Appl. Phys. 28, '89, pp 297) or "V-Groove Substrate" (M. Nagashima; Appl. Phys. Lett. 42, '83, pp 144).
However, all the techniques described above, suffer from a large restriction that special functions have to be provided additionally to optical system parts, electric circuits, etc. of optical recording/reproducing apparatus for realizing the method.
Then, a land & groove recording method of using a phase change type optical recording system has been proposed as a technique of increasing the capacity with addition of no particular functions to the apparatus (Miyagawa, et al: "Crosstalk Reducing Effect by Groove Depth Control in Land & Groove Recording", pre-text 18a-T-3 for '92 Autumn Applied Physics Association). This recording system has attracted attention as a technical means capable of doubling track density by conducting recording to both of lands and grooves of an optical recording medium instead of conducting only to one of them as in the prior art optical recording method, thereby easily increasing the capacity of information recording.
However, the land & groove recording method involves a problem regarding crosstalk and cross-erase as described below in case of further increasing the track density by reducing the track pitch thereby increasing the capacity.
At first, the crosstalk is a phenomenon, occurring upon reproduction of signals recorded in a predetermined track on a land (or groove) that signal components recorded in tracks on adjacent grooves (or lands) are read out. It has been known at present that the crosstalk can be suppressed by properly selecting the depth of the groove in the optical recording medium and that the optimal depth of the groove is reduced as the track width is narrowed as the density of the track is increased.
Meanwhile, cross-erase is a phenomenon, occurring upon recording or erasure relative to a predetermined track on a land (or a groove), that signals (marks) recorded in the tracks in adjacent grooves (lands) are partially erased. Particularly, jitter in the adjacent tracks is increased by the effect of the cross erasure and, in the worst case, this results in a disadvantage of causing reading errors for recorded signals. Increase of the jitter or causing of signal reading error may also be caused to some extent by the effect of the crosstalk. It has been known that the cross-erasure increases as the depth of the groove is reduced.
Therefore, in the land & groove recording method, if the track density is intended to be increased further, the depth of the groove has be to be reduced for suppressing the crosstalk, whereas reducing of the depth causes a conflicting problem of increasing the jitter in adjacent tracks by the effect of the cross-erase thereby deteriorating the recording characteristics. Accordingly, it has been difficult in this recording method to increase the capacity of information recording by the increased track density. By the way, it has been reported that the limit for increasing the track density in the land & groove method is 0.7 .mu.m as a track pitch (Nishiuchi, et al; "Land & Groove Recording (3) Track Pitch Dependency of Phase Change Optical Disc" in pre-text 28p-L-5 for '94 Spring Applied Physics Association).